A hydrogel contains water as the medium, so that it is useful as a gel having high biocompatibility and is used in various fields such as applications for commodities such as paper diapers, cosmetics and aromatics.
Examples of a related-art hydrogel include natural polymer gels such as agarose, and synthetic polymer gels in which between polymer chains is crosslinked through a chemical covalent bond, such as an acrylamide gel.
Recently, functional gels in which various functions such as material retention capacities, an external stimulus responsive performance and a biodegradability in consideration of the environment are imparted to a hydrogel, are attracting attention, and there are performed attempts for developing various functions by introducing functional molecules into the natural or the synthetic polymer gels using a copolymerization reaction or the like.
Thus, for imparting new functions to a hydrogel, studying the nanostructure and the surface structure of the gel in detail is required. However, the above method for introducing functional molecules using a copolymerization reaction has various problems such as problems in which the introduction rate of functional groups is limited and a precise molecule design is difficult, a safety problem of unreacted remaining materials, and further a problem in which the preparation of the gel is extremely cumbersome.
As opposed to such a related-art “top-down type” development of functional materials, there is attracting attention a “bottom-up type” study for creating functional materials by which atoms or molecules which are the minimum units of substances are assembled, and in the resultant assembly which is a supramolecule, new functions are discovered.
Also in the field of the gel, the development of a novel gel formed from a non-covalent gel fiber (so-called “nanofiber-shaped self-assembly”) produced by the self-assembly of a low molecular weight compound has been progressing. This “self-assembly” indicates such a phenomenon that in a substances (molecules) group in a random state at first, molecules associate spontaneously by an intermolecular non-covalent interaction or the like under an appropriate external condition to grow to a macro functional assembly.
The novel gel attracts attention in such a point that the control of the macroscopic structure or function of the gel is theoretically possible by controlling an intermolecular interaction or a weak non-covalent bond of a molecule assembly according to a molecule design of a monomer.
However, with respect to the way of controlling the intermolecular interaction or non-covalent bond between low molecular weight compounds, there is not yet found an apparent methodology. In addition, in the study of the non-covalent gel, because of relative easiness of the gel formation, the study of a self-assembly utilizing a hydrogen bond in an organic solvent is preceded but the study of a self-assembled compound (that is, such as a hydrogelator) in an aqueous solution remains in accidental findings.
Hydrogelators for forming a non-covalent gel which have been reported until now are broadly divided into the following three categories.
[1. Hydrogelators Having an Amphipathic Low Molecular Weight Molecule as the Skeleton Thereof]
This type of hydrogelators is created with an artificial lipid film as a model, and examples of the hydrogelators include surfactant-type gelators having a quaternary ammonium salt portion as a hydrophilic portion and having an alkyl long chain as a hydrophobic portion, and ampholytic surfactant-type gelators in which hydrophilic portions of two surfactant-type molecules are coupled.
As one example of the hydrogel formed by such gelators, there is disclosed a molecule organizational hydrogel formed by adding an anion having a molecular mass of 90 or more to a dispersion aqueous solution of a cationic amphipathic compound having a branched alkyl group in the hydrophobic portion (Patent Document 1).
[2. Hydrogelators Having a Skeleton in the Motif of Intravital Components]
Examples of this type of hydrogelators include gelators utilizing an association between molecule-assemblies through a peptide secondary structure skeleton (such as α-helix structure and β-sheet structure).
For example, there are disclosed a gelator having an α-helix structure (Non-patent Document 1) and a gelator having a β-sheet structure (Non-patent Document 2).
[3. Hydrogelators Having a Semi-Artificial Low Molecular Weight Molecule as the Skeleton Thereof]
This type of hydrogelators is composed of a combination of intravital components (hydrophilic portion), such as DNA bases, peptide chains, and sugar chains, and alkyl chains (hydrophobic portion) and the like, and can be called as a gelator combining characteristics of the above two types of gelators. Here, the DNA base, the peptide chain, and the sugar chain assume not only a role of enhancing the hydrophilicity, but also a role of imparting an intermolecular interaction such as a hydrogen bond.
For example, there are disclosed a hydrogelator containing a glycoside amino acid derivative having a sugar structure moiety having a glycoside structure of an N-acetylated monosaccharide or disaccharide (Patent Document 2), and disclosed a fine hollow fiber formed by the self-assembly from a peptide lipid of General Formula: RCO(NHCH2CO)mOH and a transition metal (Patent Document 3).
In addition, it is disclosed that an amphipathic peptide having a structure of (hydrophobic portion-cysteine residue (forming a disulfide bond during the network formation)-glycine residue (imparting flexibility)-phosphorylated serine residue-cell adhesive peptide) forms a β-sheet type fiber network with a nuclear of the hydrophobic portion (Non-patent Document 3).
In addition, there is also disclosed a case where a sugar lipid-type supramolecule hydrogel was produced using a chemical library (Non-patent Document 4).
An amphipathic dipeptide compound composed of a hydrophobic portion and a dipeptide attracts attention as one of “bottom up-type” functional materials capable of forming a self-assembly. Examples of the dipeptide compound include dipeptide compounds having a special lipid portion such as “2-(naphthalene-2-yl-oxy)acetic acid”+“glycylglycine, glycylserine or the like” which are known to become a hydrogel. However, any of these dipeptide compounds gels is produced by gelling an acidic aqueous solution, or a hydrogel produced by the gelation of any of these dipeptide compounds is acidic (Non-patent Document 5).
On the contrary, a lipid peptide compound composed of lauric acid or myristic acid which is a natural aliphatic acid and glycylglycine does not become a hydrogel and forms an organic nanotube having a hollow of multiple vesicles having an inner diameter of around 50 to 90 nm to be deposited (for example, Patent Document 3).
In addition, a lipidaminopolyol among amphipathic compounds is used as a surfactant or an emulsifier (Non-patent Document 6), however, a self-assembly formed by the self-assembly of a lipidaminopolyol (1a to 3a) described in the same document cannot form a hydrogel.